So far this happened for me as well with every flight when starting managed descent. When it started, the rate was fine but somewhere getting closer to the selected alt. it started to "chase" the purple dot.
Due to upload restrictions the images do not allow to see the following information. If required, I can provide a full resolution-version.:
Descent Start: 1000ft/m
seconds later: 5800ft/m
then: 4500ft/m ... 4300ft/m etc.
For this flight I stopped it @3800ft/m because ATC requested to change the route and therefore managed descent mode was left.
If this is the mentioned problem which is already planned to be fixed with upcoming patches, its fine, of course I/we wait. Just want to make sure + share this is not a new one.